1. Field of the Invention
The present invention relates to a ferritic stainless steel sheet which has superior workability at room temperatures and mechanical characteristics at high temperatures, and a method of producing the same. More particularly, the present invention relates to a ferritic stainless steel sheet which is suitable for use in, e.g., an automobile part in the exhaust system, specifically an exhaust manifold, which is manufactured under severe working conditions in two or more working steps, such as the steps of forming a pipe by welding, bending it and enlarging the pipe diameter, and which undergoes a load repeatedly while being heated to high temperatures of not lower than 800xc2x0 C. by exhaust gas from an engine and which is subjected to heavy vibrations from the engine, as well as a method of producing the ferritic stainless steel sheet.
2. Description of the Related Art
Ferritic stainless steel has a smaller coefficient of thermal expansion than austenitic stainless steel, and has advantages that the problem of thermal strain resulting when used in an environment subjected to high temperatures and low temperatures alternately is relatively insignificant, and that oxidation resistance at high temperatures is superior. However, ferritic stainless steel has a problem in workability when worked for shaping at room temperatures.
Various alloy elements are added to, in particular, a member used in a high-temperature environment, such as an exhaust manifold, for the purpose of increasing the strength at high temperatures. Generally, addition of various alloy elements at high rates, on one side, increases the strength at high temperatures and improves high-temperature fatigue characteristics and thermal fatigue characteristics, but on the other hand, increases the hardness and strength in working and decreases drawing formability represented by the r-value. These disadvantages make it more difficult to form a steel sheet into a complicated shape.
As one solution for overcoming the problems described above, Japanese Unexamined Patent Application No. 4-228540 proposes ferritic stainless steel in which an appropriate amount of Co is contained in Nbxe2x80x94Moxe2x80x94(Ti) added steel to improve the strength at high temperatures without causing an increase in the strength at room temperature. With the proposed ferritic stainless steel, the tensile strength (referred to as the xe2x80x9cT.S.xe2x80x9d hereinafter) at about 850xc2x0 C. increases noticeably.
With recent increasing technical demands for further improvements in eco-friendliness and fuel consumption efficiency, however, the temperature at which the exhaust manifold is employed has risen to a level over 850xc2x0 C. In other words, conventional materials are no longer adapted for such a high-temperature environment because of the insufficient strength at high temperatures.
FIG. 1 shows results of measuring changes over time in the strength (Y.S. or yield strength corresponding to a tension set of 0.2% at a strain rate of 0.3%/min) of the above-described conventional ferritic stainless steel at 900xc2x0 C.
As will be seen from FIG. 1, when the conventional steel is heated to high temperatures of 900xc2x0 C. or above, it has sufficient strength immediately after reaching such a high-temperature level. However, when holding the conventional steel at a high-temperature for a long time, the Y.S. is gradually reduced over time.
Thus, because the conventional steel does not endure a high-temperature range of 900xc2x0 C. or above for a long time, there has been a demand for development of a novel material that is highly excellent in both of strength at high temperatures and workability at room temperatures.
With a view toward satisfying the above-mentioned demand, it is an object of the present invention to provide a ferritic stainless steel sheet which has superior high-temperature fatigue characteristics, strength at high temperatures when the sheet is maintained at high temperatures for a long time, and workability at room temperatures, and to provide a method that is advantageous for producing the ferritic stainless steel sheet.
It is to be noted that the term xe2x80x9csteel sheetxe2x80x9d in this specification includes steel strips or hoops.
More specifically, the present invention is characterized as follows.
According to one aspect of the present invention, the stainless steel sheet has a composition containing, by weight,
C: not more than 0.02%, Si: 0.2 to 1.0%,
Mn: not more than 1.5%, Cr: 11.0 to 20.0%,
Ni: 0.05 to 2.0%, Mo: 1.0 to 2.0%,
Al: not more than 1.0%, Nb: 0.2 to 0.8%, and
N: not more than 0.02%,
balance essentially Fe, and an aspect ratio (dRD/dTD) of grain size in planes at xc2xc and xc2xe sheet thickness, seen in a direction normal to a sheet surface, that satisfies the following formula;
1.03xe2x89xa6(dRD/dTD)xe2x89xa61.35 
where dRD: average grain size in a rolling direction (RD direction) seen in a direction normal to the sheet surface, and dTD: average grain size in a transverse direction (TD direction) perpendicular to the RD direction seen in a direction normal to the sheet surface.
In the above ferritic stainless steel sheet, preferably, the steel sheet has a thickness of greater than 0.3 mm but not greater than 2.5 mm, and a strength Y.S.xe2x89xa6360 MPa and an r-valuexe2x89xa71.3 at 30xc2x0 C., and after maintaining the steel sheet at 900xc2x0 C. for one hour, the Y.S.xe2x89xa718.0 MPa.
In the above ferritic stainless steel sheet, preferably, P+Sxe2x89xa60.05 wt %.
Preferably, the steel sheet has a composition further containing, by weight, one or more of Ti: 0.05 to 0.5%, Zr: 0.05 to 0.5%, and Ta: 0.05 to 0.5%.
Preferably, the steel sheet has a composition further containing, by weight, Cu: 0.1 to 2.0%.
Preferably, the steel sheet has a composition further containing, by weight, one or more of W: 0.05 to 1.0% and Mg: 0.001 to 0.1%.
Preferably, the steel sheet has a composition further containing, by weight, Ca: 0.0005 to 0.005%.
According to another aspect of the present invention, there is provided a method of producing a ferritic stainless steel sheet which has superior workability at room temperatures and mechanical characteristics at high temperatures, the method comprising the steps of hot- rolling a steel ingot in a tandem rolling mill, the steel ingot having a composition containing, by weight,
C: not more than 0.02%, Si: 0.2 to 1.0%,
Mn: not more than 1.5%, Cr: 11.0 to 20.0%,
Ni: 0.05 to 2.0%, Mo: 1.0 to 2.0%,
Al: not more than 1.0%, Nb: 0.2 to 0.8%, and
N: not more than 0.02%,
a balance essentially Fe; annealing the resulting hot-rolled sheet; cold-rolling the annealed sheet once or more with intermediate annealing; and finish-annealing the cold-rolled sheet, the hot-rolling step being controlled such that the total reduction in thickness during passage through final two stands of the mill during finish hot rolling is not less than 25%, the elapsed time of passage through the final two stands is not more than 1.0 second, and the linear pressure in the final pass is not lower than 15 MN/m, the step of annealing the hot-rolled sheet being carried out at temperatures of 800 to 1050xc2x0 C., a final pass in the cold-rolling step being carried out under conditions of a sheet temperature of 80 to 200xc2x0 C. and the coefficient of friction of 0.01 to 0.2. xe2x80x9cLinear pressurexe2x80x9d denotes rolling load per unit width of the hot-rolled sheet.
In the above method, preferably, the cold rolling step is carried out such that the steel sheet has a thickness of greater than 0.3 mm but not greater than 2.5 mm.
Preferably, the steel sheet has a composition further containing, by weight, one or more of Ti: 0.05 to 0.5%, Zr: 0.05 to 0.5%, and Ta: 0.05 to 0.5%.
Preferably, the steel sheet has a composition further containing, by weight, Cu: 0.1 to 2.0%.
Preferably, the steel sheet has a composition further containing, by weight, one or more of W: 0.05 to 1.0% and Mg: 0.001 to 0.1%.
Preferably, the steel sheet has a composition further containing, by weight, Ca: 0.0005 to 0.005%.